Method for producing (meth) acrylic acid

ABSTRACT

Disclosed is a method for producing (meth)acrylic acid comprising a process of recovering (meth)acrylic acid as aqueous (meth)acrylic acid solution from a (meth)acrylic acid-containing gas mixture produced by the catalytic gas phase oxidation of at least one reactant selected from the group consisting of propane, propylene, isobutylene and (meth)acrolein, wherein the recovering process comprises the steps of: (1) feeding the (meth)acrylic acid-containing gas mixture into a quenching tower and condensing it in the quenching tower so as to recover an aqueous (meth)acrylic acid solution from the bottom of the quenching tower, in which some of the recovered aqueous (meth)acrylic acid solution is recycled to the upper portion of the quenching tower so as to condense the (meth)acrylic acid-containing gas mixture; (2) passing the uncondensed part of the (meth)acrylic acid-containing gas mixture from the top of the quenching tower to a distillation tower; and (3) heating the bottom of the distillation tower to separate water-containing impurity components from the uncondensed (meth)acrylic acid-containing gas mixture and to discharge them from the top of the distillation tower. A system used for carrying out the method is also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing (meth)acrylicacid. More specifically, the present invention relates to a method forproducing (meth)acrylic acid using a process of recovering (meth)acrylicacid as aqueous (meth)acrylic acid solution from a (meth)acrylicacid-containing gas mixture produced by the catalytic gas phaseoxidation of at least one reactant selected from the group consisting ofpropane, propylene, isobutylene and (meth)acrolein.

2. Description of the Prior Art

Conventionally, (meth)acrylic acid is obtained by the partial oxidationof propane, propylene, isobutylene and/or (meth)acrolein with aheterogeneous oxidation catalyst in the presence of water vapor. In thisoxidation method for producing (meth)acrylic acid, by-productimpurities, such as water or unreacted propane, propylene, isobutyleneand (meth)acrolein, acetic acid, formic acid, formaldehyde,acetaldehyde, maleic acid, propionic acid, furfural and the like, aregenerated. A (meth)acrylic acid-containing gas mixture containing suchby-product impurities is generally collected as (meth)acrylic acidsolution via contact with an absorption solvent, and the solvent isseparated by distillation, etc. Then, low-boiling point and high-boilingpoint components are selectively separated.

Methods for recovering (meth)acrylic acid from (meth)acrylicacid-containing gas by an absorption solvent, which have been known tillnow, can be broadly divided into methods using an organic solvent (e.g.U.S. Pat. Nos. 3,932,500 and 6,498,272) and methods using water oraqueous solution as solvent (e.g. Japanese Patent Publication No.Sho51-25602 and Japanese Laid-Open Patent No. Hei9-157213). Suchrecovering methods according to the prior art are known as methods forrecovering acrylic acid as solution from acrylic acid-containing gaswith high selectivity. There is a difference in concentration of acrylicacid in a recovered solution depending on the method used particularly.

U.S. Pat. No. 3,932,500 discloses a process comprising absorbing acrylicacid from an acrylic acid-containing product gas with a high-boiling,hydrophobic organic solvent, recovering acrylic acid from the absorbedsolution and recycling the solvent to an absorption column. In thisprocess, the concentration of acrylic acid at the bottom the absorptioncolumn is as low as 6-15% by weight, the amount of water contained inthe absorbed solution is about 5% by weight, and the concentration ofacrylic acid in the off-gas from the absorption column is about 1%. Thisloss of acrylic acid (˜1%) at the top of the absorption column isconnected directly with process economy and is burdensome consideringthat acrylic acid should be treated in subsequent processes withoutloss. Especially, in processes with larger-scale production, the loss ofacrylic acid will not be cost-efficient. In order to increase theabsorption of acrylic acid, the flow rate of a solvent for absorptionneeds to be increased. In this case, however, the concentration ofacrylic acid in a solution obtained from the bottom of the absorptioncolumn will be reduced so that the flow rate of the solvent to beseparated from acrylic acid solution in subsequent processes will beincreased, resulting in inefficiency.

Japanese Patent Publication No. Sho 51-25602 discloses a process thatcomprises absorbing an acrylic acid-containing reaction product gas withwater and recycling some of nitrogen, oxygen and water discharged froman absorption column to a reactor in order to adjust the gasconcentration required for catalytic oxidation (see FIG. 4). Thisprocess has an advantage in that the circulating supply of waterrequired in the reactor is possible since acrylic acid is absorbed withwater in the absorption column. Also, the concentration of acrylic acidat the bottom of the absorption column is 40-80% by weight, andgenerally 60-70% by weight. Furthermore, the loss of acrylic acid ventedfrom the absorption column is lower than that of the above-describedabsorption process using the organic solvent.

Other methods for recovering (meth)acrylic acid include methods in whichquenching with an acrylic acid-containing solution is combined withabsorption using water, aqueous solution or organic solvent (e.g. EP9,545 and U.S. Pat. Nos. 4,554,054 and 6,498,272). For example, acrylicacid-containing gas obtained by catalytic gas phase oxidation at a hightemperature of between 150° C. and 200° C. is quenched with acrylicacid-containing solution having a temperature of between 60° C. and 150°C. Then, uncondensed gas is discharged and recovered in a subsequentstep via absorption using a solvent. EP 9,545 discloses a method ofrecovering acrylic acid carried out in a discrete and sequential mode oran integral mode, which comprises a step of quenching of acrylicacid-containing gas and a step of absorption using water. In thismethod, the concentration of acrylic acid in aqueous acrylic acidsolution at the bottom of the recovering system is as low as about 60%by weight.

SUMMARY OF THE INVENTION

Recovering (meth)acrylic acid from product gas obtained by the catalyticgas phase oxidation as high-concentration (meth)acrylic acid solutionwith high yield can reduce the amount of by-products and impurities tobe treated in subsequent purification processes, thereby improving thecost efficiency of the recovering process. Considering the recenttendency to use a distillation process as general purification processfor (meth)acrylic acid, reducing the amount of by-products andimpurities in a distillation process consuming a large amount of energycan play an important role in improvement of cost efficiency of theprocess. Therefore, an object of the present invention is to provide amethod for recovering (meth)acrylic acid with maximized concentration.

Meanwhile, although a quenching process provides (meth)acrylic acid withrelatively high concentration, it is disadvantageous in that theconcentration of the recovered (meth)acrylic acid solution varies withquenching temperatures and a significantly small amount of (meth)acrylicacid is recovered compared to an absorption process using a solvent.Additionally, because a quenching process alone cannot accomplishcomplete recovery of (meth)acrylic acid, it should be used together withan absorption process. Under these circumstances, high-concentration(meth)acrylic acid solution obtained by a quenching process is combinedwith (meth)acrylic acid solution with relatively low concentration,which is obtained by an absorption process, resulting in a drop inconcentration of acrylic acid solution obtained by the overall process.

Therefore, another object of the present invention is to provide amethod for recovering (meth)acrylic acid by using a quenching processcombined with a distillation process, wherein (meth)acrylic acid isrecovered as (meth)acrylic acid solution with a concentration higherthan that of a (meth)acrylic acid solution obtained by a conventional(meth)acrylic acid recovering process using an organic solvent or water,so that energy and cost efficiency can be improved in purificationprocesses.

To achieve the above objects, in one aspect, the present inventionprovides a method for producing (meth)acrylic acid comprising a processof recovering (meth)acrylic acid as aqueous (meth)acrylic acid solutionfrom a (meth)acrylic acid-containing gas mixture produced by thecatalytic gas phase oxidation of at least one reactant selected from thegroup consisting of propane, propylene, isobutylene and (meth)acrolein,wherein the recovering process comprises the steps of: (1) feeding the(meth)acrylic acid-containing gas mixture into a quenching tower andcondensing it in the quenching tower so as to recover an aqueous(meth)acrylic acid solution from the bottom of the quenching tower, inwhich some of the recovered aqueous (meth)acrylic acid solution isrecycled to the quenching tower so as to condense the (meth)acrylicacid-containing gas mixture; (2) passing the uncondensed part of the(meth)acrylic acid-containing gas mixture from the quenching tower to adistillation tower; and (3) heating the bottom of the distillation towerto separate water-containing impurity components from the uncondensed(meth)acrylic acid-containing gas mixture and to discharge them from thetop of the distillation tower.

In another aspect, the present invention provides a system forrecovering (meth)acrylic acid as aqueous (meth)acrylic acid solutionfrom a (meth)acrylic acid-containing gas mixture produced by thecatalytic gas phase oxidation of at least one reactant selected from thegroup consisting of propane, propylene, isobutylene and (meth)acrolein,the system comprising: a quenching tower for condensing the(meth)acrylic acid-containing gas mixture by using an aqueous(meth)acrylic acid solution recycled to the quenching tower, thequenching tower further comprising a line for discharging an aqueous(meth)acrylic acid solution recovered from the bottom of the quenchingtower, and a line for recycling some of the recovered aqueous(meth)acrylic acid solution to the upper portion of the quenching tower;a line for passing the uncondensed part of the (meth)acrylicacid-containing gas mixture in the quenching tower through the top ofthe quenching tower to a distillation tower; a distillation tower forcarrying out distillation of the uncondensed (meth)acrylicacid-containing gas mixture by heating the bottom of the distillationtower to separate water-containing impurity components from the gasmixture; and a line for passing an aqueous (meth)acrylic acid solutionrecovered from the bottom of the distillation tower to a subsequentprocess.

The inventive method for producing (meth)acrylic acid may furthercomprise, after the process of recovering (meth)acrylic acid as aqueous(meth)acrylic acid solution from the (meth)acrylic acid-containing gasmixture produced by catalytic gas phase oxidation, a water separationprocess, a process for separating low-boiling pointcomponents/high-boiling point components, a dimer decomposition processand the like.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic process diagram showing one embodiment of thepresent invention, wherein reference letter A is a quenching tower, B isa distillation tower, and C is a stripper, and reference numeral 1 is aline for feeding a reaction product gas, 2 is a line for discharging thebottom liquid of the quenching tower, 3 is a line for discharging anuncondensed gas from the quenching tower, 4 is a line for discharginggas from the distillation tower, 5 is a line for discharging the bottomliquid of the distillation tower, and 6 is a line for feeding water forcontrolling water content of the reactor;

FIG. 2 is a process diagram showing an alternative embodiment of thepresent invention for increasing the concentration of (meth)acrylic acidin the distillation tower;

FIG. 3 is a process diagram showing another alternative embodiment ofthe present invention for removing (meth)acrolein from the aqueous(meth)acrylic acid solution recovered from the quenching tower; and

FIG. 4 shows a prior process for recovering (meth)acrylic acid using anabsorption column.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the method for producing (meth)acrylic acid in accordancewith the present invention will be described in more detail.

(a) Process for Catalytic Gas Phase Oxidation of Propane, Propylene,Isobutylene and/or (Meth)Acrolein

When propane, propylene, isobutylene and/or (meth)acrolein iscatalytically oxidized in contact with oxygen or a molecularoxygen-containing gas, such as air, a (meth)acrylic acid-containingproduct gas can be obtained.

The catalytic oxidation is conventionally carried out in two stages. Asthe first-stage catalysts, materials allowing the gas phase oxidation ofa propylene- or isobutylene-containing raw material gas and theproduction of (meth)acrolein as main product are used. As thesecond-stage catalysts, materials allowing the gas phase oxidation of(meth)acrolein-containing raw material gas and the production of(meth)acrylic acid as main product are used. The known first-stagecatalysts are oxides containing iron, molybdenum and bismuth, and thesecond-stage catalysts contain vanadium as essential component. Thetemperature of the catalytic oxidation is generally in a range of200-400° C.

In the case of production of acrylic acid from propane, propane isconverted into propylene, propylene into acrolein, and acrolein intoacrylic acid. In addition, there is another method for direct oxidationfrom propane to acrolein.

(b) Process in Quenching Tower

This is a process comprising: feeding the (meth)acrylic acid-containinggas mixture to the quenching tower A by line 1, and condensing the gasmixture in the quenching tower so as to recover an aqueous (meth)acrylicacid solution at the bottom of the quenching tower by line 2. In thisprocess, some of the recovered aqueous (meth)acrylic acid solution isrecycled to the upper portion of the quenching tower where it is used tocondense the (meth)acrylic acid-containing gas mixture.

The (meth)acrylic acid-containing gas mixture contains a large amount ofwater vapor not only produced by the catalytic oxidation as by-productbut also introduced into a reactor along with raw materials. Thus, whenthe (meth)acrylic acid-containing gas mixture is condensed in thequenching tower, some of the gas mixture will become an aqueous(meth)acrylic acid solution depending on thermodynamic properties suchas temperature and pressure, and the remainder leaves out the quenchingtower as it is. It is preferred that some of the recovered aqueous(meth)acrylic acid solution is recycled to the quenching tower withcooling so that it is used to adjust the temperature of vent gas fromthe quenching tower and to cool and condense the (meth)acrylicacid-containing gas mixture. In this regard, when the temperature of thequenching tower is increased, water contained in the gas mixture is lesscondensed, so that a relatively large amount of water can be evaporated,resulting in an aqueous solution with a high concentration of(meth)acrylic acid. If the temperature maintains low, a large amount ofwater will be condensed so that a small amount of water will beevaporated, resulting in an aqueous solution with lower concentration of(meth)acrylic acid.

The (meth)acrylic acid-containing gas mixture introduced into thequenching tower has a high temperature of 160-200° C. and thus canincrease the temperature of the quenching tower. For this reason, it ispreferred that the aqueous (meth)acrylic acid solution, which isrecycled to the quenching tower, is cooled by heat exchange in order tomaintain the temperature of the quenching tower.

The temperature of the condensed liquid in the quenching tower ismaintained at a temperature of 65-80° C., and preferably 70-78° C. Thetemperature of less than 65° C. will result in an increase in thecooling load and will show difficulty in vaporizing water, and atemperature of more than 80° C. will cause the problem of polymerizationof (meth)acrylic acid.

In substances obtained as by-products and impurities, the presence of(meth)acrolein is very critical. (Meth)acrolein which is produced mainlyin the first-stage reaction for the oxidation of propylene orisobutylene is very excellent in the ability to be polymerized, andthus, even when it is present at a very small amount, it will be easilypolymerized in a subsequent distillation process by heating, resultingin line blocking. Accordingly, it is preferred that not only(meth)acrolein but the remaining low boiling point impurities in anaqueous (meth)acrylic acid solution recovered at the bottom of thequenching tower are treated off by stripping, etc. The operation of thequenching tower at the highest possible temperature will allow theconcentration of (meth)acrolein to be maintained at a low level, but asdescribed above, make it difficult to recover (meth)acrylic acid.(Meth)acrolein in an aqueous (meth)acrylic acid solution at the bottomof the quenching tower, which is obtained at an operation condition ofabout 70° C., is about 400 ppm and can be completely removed bystripping, etc. After the (meth)acrolein is treated by stripping,(meth)acrolein and low-boiling point impurities such as water, unreactedraw materials, and gaseous by-products can be recycled to the top of thequenching tower or the gas inlet of the distillation tower so that theycan be finally discharged through the top of the distillation tower tothe outside of the system.

(c) Distillation Process

A gaseous mixture containing the remaining (meth)acrylic aciduncondensed in the quenching tower, water and inert gas such as nitrogenis discharged from the top of the quenching tower by line 3, and then issupplied to the distillation tower. While the bottom of the distillationtower is heated, impurities other than (meth)acrylic acid anduncondensed components are separated from the uncondensed (meth)acrylicacid-containing gas mixture at the top of the distillation tower. Due tothe heat supplied from the bottom of the distillation tower, watercontained in the uncondensed (meth)acrylic acid-containing gas mixture,which has a relatively high vapor pressure than (meth)acrylic acid, isevaporated preferentially, resulting in an increase in concentration of(meth)acrylic acid in the aqueous solution recovered from the bottom ofthe distillation tower.

A conventional process using a quenching tower combined with anabsorption column is different from the method using a distillationtower according to the present invention in that there is no step ofheating an absorption column for the purpose of distillation of water.

In order to heat the bottom of the distillation tower, direct heatingmethods using a kettle or siphon at the bottom of the distillationtower, or indirect heating methods using an external means (for example,a heat exchanger or reboiler) may be utilized.

The bottom temperature of the distillation tower depends on heatquantity supplied thereto. In general, the distillation tower isoperated at a bottom temperature of between 68° C. and 85° C.,preferably of 70° C. or higher, more preferably of between 72° C. and78° C. It is possible to prevent polymerization of (meth)acrylic acid toa certain degree by introducing molecular oxygen and an inhibitor.However, it is inevitable that (meth)acrylic acid forms dimer andpolymer as temperature increases. Therefore, the distillation towershould be operated at an adequate temperature determined by experimentalobservation. Since the polymerization inhibitor that may be used shouldbe water-soluble in the presence of components in the distillationtower, any inhibitor may be used as long as it is soluble in water. Inthis regard, hydroquinone generally known to one skilled in the art issufficient.

In general, when (meth)acrylic acid is recovered through an absorptionprocess wherein water or an organic solvent is brought into contact withreaction product gas in a counter-current manner, the concentration of(meth)acrylic acid in the recovered solution is 40-70% by weight (forwater) or 10-35% by weight (for an organic solvent). On the contrary,the concentration of (meth)acrylic acid in the aqueous solution obtainedfrom the method using a quenching tower combined with bottom-heateddistillation tower according to the present invention is 75-90% byweight. More particularly, it is possible to recover (meth)acrylic acidwith a very high concentration in the form of the aqueous (meth)acrylicacid solution containing 75-90% by weight of (meth)acrylic acid, 1-4% byweight of acetic acid, 0.2-0.7% by weight of various high-boilingimpurities and 8-20% by weight of water. Because the concentration of(meth)acrylic acid in the aqueous (meth)acrylic acid solution isincreased according to the present invention, the amount of water asimpurity to be treated in a subsequent step is reduced, resulting insaving of energy consumption needed to such treatment. Furthermore,various types of purifying processes may be selected. For example, whenthe amount of water in the aqueous (meth)acrylic acid solution is low,it is possible to recover (meth)acrylic acid directly through acrystallization process rather than a conventional distillation processand to select a process using membrane separation technique with verylow energy consumption.

Meanwhile, during the evaporation of water with a lower boiling pointthan that of (meth)acrylic acid, caused by heating the bottom of thedistillation tower for increasing concentration of (meth)acrylic acid inthe aqueous solution in the bottom of the distillation tower, it isexpected that (meth)acrylic acid having high affinity to water is alsodischarged from the top of the distillation tower along with water,resulting in loss of (meth)acrylic acid. To obtain high yield of(meth)acrylic acid from the (meth)acrylic acid-containing reactionproduct gas, the amount of (meth)acrylic acid discharged along withwater should be decreased. To accomplish this, it is preferable thatvaporous (meth)acrylic acid moving upwardly toward the top of thedistillation tower is caused to be moved into liquid water, which issupplied from the top part of the distillation tower and is movingdownwardly to the bottom of the distillation tower, via sufficientgas-liquid contact and mass transfer, so that the (meth)acrylic acid canbe obtained as aqueous solution at the bottom of the distillation tower.Without such liquid supply to the top part of the distillation tower,the only means, i.e., heating at the bottom of the distillation towerwhile passing uncondensed gas upwardly from the bottom of thedistillation tower cannot accomplish the mass transfer via gas-liquidcontact. Therefore, it is preferable that a small amount of water issupplied to the top part of the distillation tower as reflux forconstituting the distillation tower so that counter-current gas-liquidcontact can be made while permitting distillation. Additionally, asdescribed in Japanese Patent Publication No. Sho51-25602, uncondensedpart of the gas discharged from the top of the distillation tower, whichcontains nitrogen, oxygen, unreacted propylene, isobutylene and(meth)acrolein, (meth)acrylic acid and water may be recycled to thereactor. Among those components, nitrogen and water are important. It isessential in a commercial production process that nitrogen supplied in alarge amount is recycled. Additionally, water is one of the essentialcomponents of the catalytic oxidation, and thus one-pass water supply inthe form of water vapor to reactor directly is not cost-efficient.Therefore, energy saving can be accomplished by using recycled waterflow in a significant part of water supply. In this case, it ispreferable to adjust the water content in the gas discharged from thetop of the distillation tower to 15-30% by volume so as to provide asufficient water supplying source. To satisfy this, water issupplemented at the top of the distillation tower to compensate forinsufficient water supply by using the heat supplied from thedistillation tower.

Although water supplied to the top of the distillation towersubstantially returns to the reactor to serve to adjust water content,the recycled amount of water should not be excessively large. Aftercalculating incoming and outgoing energy, heat quantity needed forvaporizing the water supplied to the top of the distillation towerultimately results from the reboiler disposed at the bottom of thedistillation tower and consumption of steam supplied to the reboiler.Therefore, in order to reduce consumption of steam, a suitable amount ofwater should be supplied. We have observed that the amount of watersuitably ranges from 15 to 30% by volume of the gas flow discharged fromthe top of the distillation tower. It is more economically preferablethat the amount of water ranges from 19 to 25% by volume of the gas flowdischarged from the top of the distillation tower. Additionally, thetemperature of the top of the distillation tower ranges from 55° C. to68° C. so as to satisfy such water content.

Water contained in the aqueous solution recovered from the bottom of thedistillation tower and that of the quenching tower is recovered bypassing it through a subsequent water separation process. A part of therecovered water is subjected to conventional wastewater disposal orpartially recycled to the distillation tower. Preferably, water supplyto the top of the distillation tower is controlled in such a manner thatthe supplemented amount of water plus the amount of fresh processingwater and recycled water can be present in the above volume percentrange.

After operation of the distillation tower at the bottom temperature of75° C., the top temperature is 60-68° C., water content in the off-gasis 20-25% by volume and acrylic acid concentration in the off-gas is0.5-0.9% by volume, the top temperature varying with the amount ofwater.

The distillation tower that may be used includes a conventional platetower, wetted-wall tower, packing tower, etc. Generally, a plate toweror packing tower is preferable, a packing tower being the mostpreferable.

In order to make sufficient gas-liquid contact, high-efficiency packingis used preferably. Various kinds of packing may be used for providingsufficient gas-liquid contact and non-limiting examples thereof includesheet-type packing such as gauze packing or Mellapak, grid-type packingsuch as Flexigid, random packing such as Raschig ring, Pall ring orCascade mini ring, etc. Preferably, the packing is selected consideringmass transfer, pressure gap between the top and bottom of thedistillation tower, or the like. We have found that structured packingsprovide the most preferable results.

Aqueous (meth)acrylic acid solution in the bottom of the distillationtower essentially comprises (meth)acrylic acid, water, and a smallamount of by-product impurities (e.g. acetic acid), and the particularcomposition depends on the bottom temperature. When the distillationtower is operated in the above range of bottom temperatures,(meth)acrolein, which tends to cause problems such as line blocking, issubstantially discharged from the top of the distillation tower. Theconcentration of (meth)acrolein in the aqueous (meth)acrylic acidsolution obtained from the bottom of the distillation tower is 100 ppmor less with the proviso that the bottom temperature of the distillationtower is 75° C. In this case, there is no need for treatment oflow-boiling point materials (for example, treatment of the aqueoussolution recovered from the bottom of the distillation tower by astripper). To accomplish flexible operation depending on variations inoperational conditions, it is possible to selectively use or skip astripper based on the concentration of (meth)acrolein obtained byanalytical works.

Then, the aqueous solution recovered from the bottom of the distillationtower as well as the aqueous solution obtained by treating the aqueoussolution from the bottom of the quenching tower by a stripper canprovide purified (meth)acrylic acid by subsequent (meth)acrylic acidpurification processes, including water separation, light and heavy cutsseparation, and thermal decomposition. These purification processes maybe generally carried out by conventional methods.

Hereinafter, a kind of embodiment of the present invention will bedescribed with reference to the accompanying drawings.

FIG. 1 is a schematic diagram showing a system for recovering(meth)acrylic acid as aqueous (meth)acrylic acid solution from a(meth)acrylic acid-containing gas mixture produced by catalytic gasphase oxidation, according to one embodiment of the present invention.

First, a reaction product gas obtained by the catalytic gas phaseoxidation of propane, propylene, isobutylene and/or (meth)acrolein withmolecular oxygen is fed to quenching tower A through line 1. Thesolution obtained from the bottom of the quenching tower is cooled in aheat exchanger in a forced-circulation manner and then subjected torecycling to the upper portion of the quenching tower so that hotreaction product gas can be cooled. Through line 2, an aqueous(meth)acrylic acid solution from the bottom of the quenching tower ispassed to units for separating and purifying (meth)acrylic acid insubsequent processes. As the subsequent processes, stripping forlow-boiling point impurities such as (meth)acrolein and/or aldehydes andseparation methods for purifying (meth)acrylic acid, such asdistillation, crystallization and membrane separation may be used.

The gas containing the remaining (meth)acrylic acid which has not beencondensed in the quenching tower is passed to distillation tower Bthrough line 3. To the top of the distillation tower, water is suppliedthrough line 6 for controlling water content in the reactor, and inertand uncondensed gaseous mixture discharged from the distillation toweris recycled to the reactor through line 4 or passed to the waste gascatalytic incinerator system (WGCIS).

Water is discharged from the top of the distillation tower by heatingthe bottom of the distillation tower with a reboiler, thereby increasingthe concentration of (meth)acrylic acid in the aqueous (meth)acrylicacid solution. The aqueous (meth)acrylic acid solution obtained therebyis discharged through line 5 and passed to a subsequent (meth)acrylicacid purification process.

FIG. 2 illustrates an alternative embodiment for increasing theconcentration of (meth)acrylic acid in the aqueous (meth)acrylic acidsolution by enhancing evaporation of water in the distillation tower,wherein a part of the aqueous (meth)acrylic acid-containing solution,which is recovered from the bottom of the distillation tower, ispartially discharged and another part thereof is recycled to anyposition of the distillation tower in the presence of heat supply.

FIG. 3 illustrates another alternative embodiment using treating unit Cfor removing low-boiling point materials such as (meth)acrolein from theaqueous (meth)acrylic acid solution recovered from the quenching tower.The flow discharged from treating unit C is recycled to the top part ofthe quenching tower through line 7 and then discharged to the outside ofthe system through line 8.

The embodiments in the drawings illustrate the respective methods, andthese methods may also be performed in combination. Hereinafter, thepresent invention will be described in more detail by way of examples,but it is to be understood, however, that these examples are notconstrued to limit the scope of the present invention.

COMPARATIVE EXAMPLE 1

A reaction product gas obtained by the catalytic gas phase oxidation ofpropylene with a molecular oxygen-containing gas was introduced into aconventional absorption column as shown in FIG. 4 and acrylic acidcontained therein was absorbed and collected by using water. Thecomposition of the reaction product gas consisted of 70.5% by weight ofuncondensed components of nitrogen+oxygen, 1.5% by weight of unreactedpropylene+propane, 2.8% by weight of carbon dioxide+carbon monoxide,9.5% by weight of water, 14.5% by weight of acrylic acid, and theremainder of other condensable components.

As absorption column unit, a tray column with an internal diameter of200 mm was used, and the reaction product gas was cooled to 170° C. witha heat exchanger placed on the exit line of an oxidation reactor and fedto the bottom of the absorption column. The acrylic acid-containingsolution obtained from the column bottom was circulated to the fifthstage from the bottom through a line on which the external heatexchanger for cooling the circulating solution was placed. The columnconsisted of a total of 25 stages, and water with a temperature of 55°C. was fed to the top of the column. The absorption column was operatedat a top temperature of 60° C. under a pressure of 1050 mmH₂O. At thetop of the absorption column, 70% by weight of water based on the weightof acrylic acid contained in the reaction product gas was introduced toabsorb the acrylic acid. The composition of the aqueous acrylic acidsolution at the column bottom, which had been recovered as describedabove, contained 61.8% by weight of acrylic acid, the loss of acrylicacid to the top of the column was 1.8% by volume, and water content was18.4% by volume.

EXAMPLE 1

A reaction product gas of acrylic acid with the same composition asillustrated in Comparative Example 1 was used. As quenching tower, anSUS ring-packed drum with a diameter of 300 mm and a height of 80 mm wasused, and a portion of the solution at the bottom of the quenching towerwas circulated to the top of the quenching tower through a line on whicha heat exchanger was placed so that the temperature of the solution atthe column bottom reached 72° C. The composition of gas discharged fromthe top of the quenching tower consisted of 82.5% by weight ofnitrogen+oxygen, 12.5% by weight of acrylic acid, 9.8% by weight ofwater and the balance amount of impurities, and had a temperature of61.5° C. The gas discharged from the quenching tower was passed throughan insulated line to a distillation tower. As the distillation tower, agauze-packed column, which has a diameter of 200 mm and a height of 1350mm, similar with one illustrated in Comparative Example 1 was used. Tothe top part of the distillation tower, water was supplied at 55° C.Particularly, the water supply was controlled so that the water contentin the gas discharged from the top of the distillation tower, which wascollected and analyzed during the operation of the system, can be 24% byvolume. The top temperature and pressure in the distillation tower wereset to 65° C. and 1050 mmH₂O, respectively. A 3 L flask was placed atthe bottom of the distillation tower and the flask was heated to adjustthe temperature of the bottom liquid to 75° C. The composition of theaqueous acrylic acid solution recovered from the bottom of the quenchingtower contained 79.2% by weight of acrylic acid and that recovered fromthe bottom of the distillation tower contained 72% by weight of acrylicacid. Additionally, the gas discharged from the top of the distillationtower contained 0.9% by volume of acrylic acid.

EXAMPLE 2

Example 1 was repeated, except that the aqueous acrylic acid solutionwas pumped out from the flask placed at the bottom of the distillationtower, heated to 80° C. by using a heat exchanger, and 75% by weight ofthe solution was recycled to the cascade mini ring layer packed to aheight of 50 cm from the bottom of the distillation tower. The toptemperature of the distillation tower was 66° C. Concentration ofacrylic acid in the off-gas was 1.1% by volume, and that in the aqueousacrylic acid solution recovered from the bottom of the distillationtower was 75% by weight.

INDUSTRIAL APPLICABILITY

As described above, according to the inventive method for producing(meth)acrylic acid by using a quenching process combined with adistillation process while heating the bottom of the distillation tower,it is possible to recover (meth)acrylic acid as aqueous solutioncontaining (meth)acrylic acid in a higher concentration compared to a(meth)acrylic acid solution obtained by a conventional method forrecovering (meth)acrylic acid by using an organic solvent or water,resulting in a reduction in the cost of operating energy and equipmentinvestment for the subsequent separation processes. This allows the(meth)acrylic acid production process to be efficient and economic.

1. A method for producing (meth)acrylic acid comprising a process ofrecovering (meth)acrylic acid as aqueous (meth)acrylic acid solutionfrom a (meth) acrylic acid-containing gas mixture produced by thecatalytic gas phase oxidation of at least one reactant selected from thegroup consisting of propane, propylene, isobutylene and (meth)acrolein,wherein the recovering process comprises the steps of: (1) feeding the(meth)acrylic acid-containing gas mixture into a quenching tower andcondensing it in the quenching tower so as to recover an aqueous(meth)acrylic acid solution from the bottom of the quenching tower, inwhich some of the recovered aqueous (meth)acrylic acid solution isrecycled to upper portion of the quenching tower so as to condense the(meth)acrylic acid-containing gas mixture; (2) passing the uncondensedpart of the (meth)acrylic acid-containing gas mixture from the top ofthe quenching tower to a distillation tower; and (3) heating the bottomof the distillation tower to separate water-containing impuritycomponents from the uncondensed (meth)acrylic acid-containing gasmixture and to discharge them from the top of the distillation tower. 2.The method as claimed in claim 1, wherein the temperature of thesolution condensed in the quenching tower ranges from 65° C. to 85° C.3. The method as claimed in claim 1, wherein the temperature of thebottom of the distillation tower ranges from 68° C. to 85° C.
 4. Themethod as claimed in claim 1, wherein a small amount of water issupplied to the top part of the distillation tower as reflux so as tomake counter-current gas-liquid contact that permits distillation in thedistillation tower.
 5. The method as claimed in claim 4, wherein theamount of water supplied to the top part of the distillation tower iscontrolled in such a manner that the water content in the gas dischargedfrom the top of the distillation tower can be 15-30% by volume.
 6. Themethod as claimed in claim 1, wherein a part of the aqueous(meth)acrylic acid solution discharged from the bottom of thedistillation tower is heated and recycled to any point of thedistillation tower.
 7. The method as claimed in claim 1, wherein theconcentration of (meth)acrylic acid in the aqueous (meth)acrylic acidsolution obtained from the quenching tower is 75% or higher and theconcentration of (meth)acrylic acid in the aqueous (meth)acrylic acidsolution obtained from the distillation tower is 65% or higher.
 8. Themethod as claimed in claim 1, wherein the temperature of the top of thedistillation tower ranges from 55° C. to 68° C.
 9. The method as claimedin claim 1, wherein the aqueous (meth)acrylic acid solution recycled tothe quenching tower in step (1) is cooled by heat exchange.
 10. Themethod as claimed in claim 1, wherein either or both of the bottomliquid of the quenching tower and the bottom liquid of the distillationtower are treated by a stripper to separate (meth)acrolein.
 11. Themethod as claimed in claim 10, wherein the gas treated by the stripperand discharged from the stripper is supplied to the quenching tower ordistillation tower in a condensed or uncondensed state.
 12. A system forrecovering (meth)acrylic acid as an aqueous (meth)acrylic acid solutionfrom a (meth)acrylic acid-containing gas mixture produced by thecatalytic gas phase oxidation of at least one reactant selected from thegroup consisting of propane, propylene, isobutylene and (meth)acrolein,the system comprising: a quenching tower for condensing the(meth)acrylic acid-containing gas mixture by using an aqueous(meth)acrylic acid solution recycled to the quenching tower, thequenching tower further comprising a line for discharging an aqueous(meth)acrylic acid solution recovered from the bottom of the quenchingtower, and a line for recycling some of the recovered aqueous(meth)acrylic acid solution to the upper portion of the quenching tower;a line for passing the uncondensed part of the (meth)acrylicacid-containing gas mixture in the quenching tower through the top ofthe quenching tower to a distillation tower; a distillation tower forcarrying out distillation of the uncondensed (meth)acrylicacid-containing gas mixture by heating the bottom of the distillationtower to separate water-containing impurity components from the gasmixture; and a line for passing an aqueous (meth)acrylic acid solutionrecovered from the bottom of the distillation tower to a subsequentprocess.